| 摘要 |
An electron gun having: a cathode for emitting electrons; a first Wehnelt electrode equipped with a first aperture through which electrons are allowed to pass; and a second Wehnelt electrode that is equipped with a second aperture disposed at a predetermined position with respect to the cathode and the first aperture, and that is furnished at a position closer to the cathode than the first Wehnelt electrode, wherein: the cathode and the second Wehnelt electrode are included within a single assembly constituting a unitary body; and the assembly is detachably attached to the first Wehnelt electrode. Replacement of the cathode can be performed by detaching the cathode unit from the first Wehnelt electrode, and then ejecting the cathode unit out from the Wehnelt cover. The emitter of the cathode can thereby be reliably positioned with respect to the second aperture. |
